WO2001046658A1

WO2001046658A1 - High-resolution littrow spectrometer and method for the quasi-simultaneous determination of a wavelength and a line profile

Info

Publication number: WO2001046658A1
Application number: PCT/EP2000/012969
Authority: WO
Inventors: Helmut Becker-Ross; Stefan Florek; Michael Okruss
Original assignee: Gesellschaft zur Förderung angewandter Optik, Optoelektronik, Quantenelektronik und Spektroskopie e.V.; Gesellschaft zur Förderung der Spektrochemie und angewandten Spektroskopie e.V.
Priority date: 1999-12-20
Filing date: 2000-12-19
Publication date: 2001-06-28
Also published as: US6717670B2; DE19961908A1; DE19961908C2; US20020180969A1; AU2674901A

Abstract

The invention relates to a spectrometer (10) with a dispersive element (16) that can be displaced between at least two positions. In the first position, the simply dispersed radiation (44) of a selected wavelength is reflected directly back in the incident beam path (42), while in the second position the dispersed radiation (32) of the selected wavelength can be directed to a reflective element (30) that is positioned such that the radiation (34) can be directed at least one more time across the dispersive element (16) and then back to the incident beam path (38). The spectrometer is provided with a device, for example, a mirror, an echelle grating or a prism that deflects the beam from the plane of dispersion, which is arranged in such a manner that the simply dispersed beam (34) runs in another plane than the multiply dispersed beam (36). The mirror (30) is inclined by an axis (54) that extends parallel to the plane of dispersion and perpendicular to the incident beam (32).

Description

High-resolution Littrow spectrometer and method for quasi-simultaneous determination of a wavelength and a line profile

Technical field

The invention relates to a spectrometer with a dispersing element which is movable between at least two positions and in which in the first position the simply dispersed radiation of a selected wavelength falls directly back into the incident beam path.

The invention further relates to a method for measuring the radiation from a spectral range with different spectral resolution, in which the beam is directed from an imaging optics onto a dispersing element and from which

Dispersion element back to the imaging optics. The invention relates specifically to the use of such a spectrometer. State of the art

The spectrometers described under the preamble of claim 1 are also known under the term Littrow spectrometer. With a Littrow arrangement, the

Beam of a selected wavelength approximately back into itself after dispersion on a grating or prism. As a result, the same optical components can be used as imaging and camera optics. A complete superimposition of the incident and dispersed rays is practically not possible since no detector can be arranged in the entrance slit. A deviation is generally only tolerable for small angles, since the aberrations increase.

High-resolution spectrometers with spectral bandwidths that are up to 15 f narrow are used in particular when measuring the spectral intensity distribution of lasers. In addition to measuring the spectral intensity distribution, it may also be desirable to determine the exact spectral position of the intensity maximum or the center of the line. For this purpose, the position of the detectors or the image points on the detector must be precisely assigned to the respective wavelengths.

Echelle spectrometers are known which contain gratings with a step-like cross section. A diffraction pattern is generated by the step-like structure with a corresponding blaze angle, which diffuses the intensity in a very high order, e.g. concentrated in the eightieth to one hundredth order. These orders can overlap - depending on the incident wavelengths.

With some such Echelle spectrometers, the orders are therefore dispersed again perpendicular to the dispersion plane in order to separate the different orders that occur. This gives a two-dimensional spectrum that can be detected with line or area detectors. Echelle spectrometers from the publication "Novel metrology for measuring spectral purity of KrF lasers for deep UV lithography" by AIErshov, GGPadmabandu, J.Tyler, P.Palash in http://www.cymer.com from 17.3.1999 are still available known for lasers in which a collimated beam is directed onto a grating in a Littrow arrangement. In the Littrow arrangement, the beam of a certain wavelength is reflected at the grating at an angle such that it at least approximately falls back into itself.

The dispersed beam is directed onto a line detector by means of a mirror. There is a partially transparent mirror between the collimator lens and the grating. As a result, part of the light after the first dispersion on the grating is reflected on the partially transparent mirror and redispersed on the grating. The semitransparent mirror is positioned so that the perpendicular forms a small angle with the beam. This results in a slight angular displacement of the beams with single and multiple dispersion, so that the peaks are at different locations on the detector.

The known arrangement uses an intensity-weakening partially transparent mirror, which must also be passed through twice by the simply dispersed beam. In the case of multiple runs, the intensity is correspondingly reduced more. To measure with a single pass without such a weakening, the partially transparent mirror must be removed from the beam path.

Disclosure of the invention

The invention has for its object to provide a high-resolution spectrometer with which both the spectral intensity distribution of an emission line relative to the center of gravity and the absolute wavelength position of the center of gravity can be determined with high accuracy and sensitivity.

According to the invention the object is achieved in that in the second position the dispersed radiation of the selected wavelength on a reflective element falls, which is arranged so that the radiation can be directed at least one more time over the dispersing element and then back into the incident steel passage.

With such an arrangement, the dispersing element can be adjusted so that the beam is directed back directly. By changing the position e.g. a simple rotation of the dispersing element, however, the beam can also be directed onto a reflecting element, from which it is directed back onto the dispersing element. Then the beam runs twice over the dispersing element. Accordingly, a higher spectral resolution is achieved. Since the beam is not split, a good signal-to-noise ratio is maintained even with multiple passes. Only the loss of reflection and efficiency at the reflecting and dispersing element reduce the intensity.

To separate the radiation according to the number of passes, means for deflecting the beam from the dispersion plane can be provided. They are dimensioned such that the single-dispersed jet runs on a different plane than the multiple-dispersed jet. A mirror that is inclined about an axis that runs parallel to the dispersion plane and perpendicular to the incident beam is particularly suitable for this. The deflection can also be done with a prism.

In a preferred embodiment, the dispersing element is a grating, which in an especially preferred embodiment is an Echelle grating. The blaze angle of the Echelle grating is preferably at least 45 °. Then measurements are carried out in high order at a large angle of incidence, whereby a high resolution is achieved according to the grid equation.

An Echelle spectrometer according to the invention, in which the positions of the grating are determined by the angle to the incident beam, has the advantage that the two positions can be achieved by a simple grating rotation. The axis of rotation is the same as the axis about which the grating must be rotated for wavelength adjustment. According to the invention, a mirror which is designed as a plane mirror or a prism which is mirrored on one side can serve as the reflecting element. The mirror or the prism are preferably used at the same time to deflect the beam from the dispersion plane. In the case of a mirror, this is preferably done by tilting by a very small angle about an axis which is parallel to the dispersion plane and runs perpendicular to the beam falling on the mirror. As a result, the multiply dispersed beam is deflected somewhat upwards or downwards and appears offset in height from the simply dispersed beam in the image plane, on which a detector is usually arranged.

The reflective element is advantageously arranged in such a way that the angle which the perpendicular to the dispersing element forms with the dispersed beam is smaller than the angle with the incident beam. As a result, the parallel beam that falls on the grating has a smaller diameter than the beam that falls on the reflecting element. The reflective element, e.g. the plane mirror is then larger than the collimator mirror, which is generally more expensive. However, there are also arrangements in which the reverse arrangement is more advantageous.

Means for controlling the angle at which the incident beam falls on the dispersing element are preferably provided on the spectrometer. These means can be formed by a stepper motor with which a lever can be moved, with which a rotation of the dispersing element is brought about.

Means for controlling the angle at which the dispersed jet leaves the dispersing element are preferably also provided. Means are also provided with which the positions of the components can be controlled from a computer. Then the grating can be rotated from the computer without further modification of the spectrometer and thus the wavelength and the number of grating passes can be set. In a preferred embodiment of the invention, an entry slit is provided, as well as means with which the beam can be deflected in such a way that the primary image of this entry slit lies laterally offset next to the entry slit at a selected wavelength in the dipersion plane. This lateral displacement can be done by simply turning the

Grid done at a small angle. The dispersion plane is generally defined as the plane perpendicular to the lattice furrows when the dispersing element is represented by a lattice. If the dispersing element is represented by a prism, then the dispersion plane is defined as the plane that is perpendicular to the roof edge of the prism.

In an alternative embodiment of the invention, means are provided with which the beam can be deflected such that the primary image of this entry slit is offset at a selected wavelength perpendicular to the dispersion plane above or below the entry slit. Such a shift can be done by tilting an imaging

Elements, e.g. a mirror.

In addition to the dispersing element, only reflective optical components are preferably provided in the spectrometer. This avoids chromatic aberrations and the spectrometer can be used in any

Wavelength range can be used. A subsequent enlargement e.g. By means of two cylinder mirrors only in the direction of the dispersion plane, the resolution of the device is not limited by the finite width of the detector elements.

According to the invention, in order to measure the radiation from a spectral range with different spectral resolution, the beam is directed from an imaging optics to a dispersing element and from the dispersion element back to the imaging optics. The beam is then optionally directed by a change in position of the dispersing element, for example a rotation, onto a reflecting element, from which it is directed back onto the dispersing element. You can use the following procedure:

(a) Positioning the dispersing element such that the dispersed beam of a selected wavelength travels back into itself

(b) measuring the signal on a detector,

(c) positioning the dispersing element so that the dispersed beam of a selected wavelength falls on the reflecting element, and

(d) Measuring the signal at the detector.

If, after the second positioning of the dispersing element, the light of a reference light source is coupled into the spectrometer, then the smaller resolution with a large inspection area can be used to assign the wavelengths to the detector elements by means of a reference light source. The reference light source, e.g. a low pressure hollow cathode lamp is generally considerably less intense than a laser, for example. If the line radiation of the reference light source is then distributed over too many detector elements, a bad signal-to-noise ratio is obtained for each detector element. For the determination of the

Line peaks or the center of gravity may be sufficient a lower resolution. So it makes sense to measure here with a simple grid passage.

However, when measuring a line profile from a laser, the intensity is sufficient and the line can be distributed over many detector elements. Here the grating can be rotated accordingly so that measurements can be carried out with multiple grating passes. Of course, the reference radiation can also be measured first.

A spectrometer according to the invention is particularly suitable for determining the spectral properties of an excimer laser for photolithography. The spectral properties of the excimer laser can thus be used for its control and regulation.

In photolithography, the line profile of the laser used can be used Determine laser quality. If, for example, the spectral half-width in a KrF excimer laser is greater than a threshold value, the gas mixture must be exchanged. Peak to peak fluctuations of the laser pulses can also be observed and evaluated, so that only those pulses that actually meet certain criteria are actually directed onto the wafer. With the spectrometer according to the invention, a resolution can be achieved which allows simultaneous line profile measurement and thus control and regulation of the lithography process based on the spectral properties of the laser.

Embodiments of the invention are the subject of the dependent claims. Some exemplary embodiments of the invention are described below with reference to the accompanying drawings. Show in it

Fig.l The spectrometer according to the invention with double grating passage, laterally offset exit slit and fixed mirror positions

Fig.2 The structure of an Echelle grid.

Fig.3 The spectrometer according to the invention with double grating passage, tiltable parabolic mirror and laterally offset exit slit.

Fig.4 The spectrometer according to the invention with a simple lattice passage and an outlet slit offset perpendicular to the dispersion plane.

5 shows the spectrometer according to the invention with a double lattice passage and an exit slit offset perpendicular to the dispersion plane.

Fig.6 The spectrometer according to the invention with a tilted reflector.

Fig.7 The spectrometer according to the invention with reduced external dimensions and re-enlargement. Fig. 8 A detector signal of a fully resolved double line at twice

Grid passage.

Fig. 9 A detector signal of the incompletely resolved double line from Fig. 8 with a single grid pass.

Description of the embodiments

In Fig.l 10 is a spectrometer. The light is coupled into the spectrometer 10 via an entry slot 12. At an off-axis parabolic mirror 14, the divergent beam is collimated into a parallel bundle and directed onto an Echelle grating 16 with a large blaze angle of at least 45 degrees.

An Echelle grating is shown again in detail in FIG. 2. The Echelle grating comprises step-shaped grating furrows. The lattice furrows have a distance d which corresponds to the lattice constant. The angle of incidence α is the angle between the incident beam 20 and the perpendicular 22 on the grating 16. The angle of incidence β is the angle between the reflected beam 24 and the perpendicular 22 on the grating 16.

The blaze angle Θ _B is the angle between the vertical 22 on the grid 16 and the vertical on the grid furrows 18.

A high blaze angle concentrates the intensity of the diffraction image on high diffraction orders in the region between the eightieth and hundredth orders. A high diffraction order results in a high resolution. Likewise, a large diffraction angle causes high resolution. The Echelle grating has a high line count of 50 to 100 lines per millimeter in order to make the diffraction-limited resolution as large as possible. Due to the large angle of incidence, the grating must be correspondingly long if all the radiation is to strike the grating. In Fig.l an axis of rotation is shown at 26. The grid 16 is arranged rotatably about this axis 26, as is illustrated by an arrow designated by 28. Rotation can be accomplished by mounting the grille on a turntable (not shown) connected to a lever. The lever can be moved from a computer by means of stepper motors and thus a defined rotation can be brought about. The wavelength to be measured or the wavelength range can be selected in a particularly simple manner via the angle of rotation of the grating.

The grating 16 is set so that the dispersed beam falls on a plane mirror 30 for the selected wavelength. This is shown by arrow 32. The

The plane mirror 30 is positioned in such a way that the reflected beam directly falls back into itself and hits the grating 16 again. This is shown by arrow 34. This causes a double grating passage, which results in an increased spectral resolution. At the set angle on the grating 16, the beam 38 falls back into itself. This is shown by arrow 36. In this case, the beam 38 does not fall back completely, but only to the extent that the dimensions of the entry slit 12 permit it, so that the detector can also be arranged next to it.

The beam is refocused by the parabola 14 and directed just past the entry slit 12, where the spectrum in an exit plane 40 with a detector line

Detector is detectable. A CCD line can be used as the detector, but any other detector line or other detector array is also suitable that has a sufficiently small pixel size.

3 shows the spectrometer 10 from FIG. 1 with a modification. The grid

16 is arranged so that the dispersed beam, which is represented by an arrow 44, is reflected back directly into itself, ie into the incident beam, which is represented by an arrow 42, and does not strike the mirror 30. The spectrometer 10 has a correspondingly lower spectral resolution than in a grating position in which a double pass is effected. By rotating the parabolic mirror 14 about an axis 46, which is represented by an arrow 48 in FIG. 3, the beam can be displaced a little in the plane. Then the spectrum does not appear next to, but above or below the entrance slit 12. This offset occurs both with a single lattice pass, as shown in FIG. 4, and also with a multiple lattice pass, as shown in FIG. 5. The returning beam, represented by an arrow 50, therefore runs above the incident beam, represented by an arrow 52.

There are grating positions in which the corresponding wavelength runs back directly to the parabolic mirror 14 in one order, but precisely to the mirror 30 in another order. The latter are reflected back in the manner described above and run again over the grating. Then one finds two different signals from different orders in the image plane of the primary image 40. At least one of these signals is undesirable.

This problem can be solved by tilting the mirror 30 about an axis 54 which is parallel to the plane of dispersion and perpendicular to the incident beam. The tilt angle can be very small. This minimizes the imaging errors that arise. However, it must be so large that the image 58 of the entry slit 12 in the primary image plane is above or below the image 60 of the entry slit 12 in the case of a single dispersed jet in the case of a multiply dispersed jet. 6, the image 58 of the multiply dispersed jet lies below the image 60 of the single dispersed jet.

Instead of a tilted plane mirror 30, a correspondingly arranged Littrow

Suitable for prism (not shown). Due to the one-sided mirrored prism with a correspondingly small prism angle, the beam is slightly bent out of the dispersion plane of the grating 16. The image 58 or 60 of the entry gap 12 is then also offset in the exit plane on the detector.

7 shows a spectrometer according to the invention in which the external dimensions have been reduced. The light is in the by means of a light guide 62 Entry gap 12 passed. This allows the light source to be positioned anywhere. Flat mirrors 64 and 66 in the beam path cause the beam path to fold. The beam path is also folded by means of cylinder mirrors 68 and 70. The cylindrical shape causes the image to be enlarged in the direction of the dispersion, while the dimensions of the image perpendicular to the direction of dispersion remain the same.

As a result, a peak is distributed over more picture elements of the detector and the picture elements are no longer resolution-limiting.

With the structure described, two different measurements with different resolution and different intensity can be carried out on a detector element by simply rotating the grating 16 without further changes. At the first grating position, the beam is dispersed twice and only then runs back onto the detector 74. This achieves a high resolution and the intensity of the light is distributed over many detector elements. This setting is therefore particularly suitable for measuring Linienpro filen of high-intensity light sources, such as laser light sources. A signal 80 obtained with this setting is shown in FIG. You can see two clearly separated peaks 82 and 84.

In a second grating position, the simply dispersed beam is returned directly to the detector 74. This results in a somewhat lower resolution and distributes the light to fewer detector elements 72. The signal-to-noise ratio at each detector element is thereby greater. This setting is particularly suitable for measuring the emission spectrum of low-intensity light sources, such as low-pressure hollow cathode lamps. Then the line profile of the emission peaks may not be completely resolved, but the resolution is still sufficient to determine the center of gravity of the line or the maximum intensity.

A signal 86 obtained with this setting from the same light source as in FIG. 8 is shown in FIG. You can see a double peak on a significantly reduced one

Background noise. The lower noise is due to the fact that the same

Intensity due to the lower resolution distributed to fewer detector elements and on the other hand because the reflection losses at the mirror and especially at the grating do not occur. However, the signal is sufficient to determine, for example, a line center of gravity with good accuracy.

The combination of the two measurements described above offers particular advantages: With the high resolution, the line profile can be measured. With the low resolution, the wavelength of a reference light source, such as, for example, a hollow cathode lamp, can be determined and the wavelength calibration of the detector elements can thus be carried out.

The grating position relative to the position of the off-axis parabolic mirror can be determined using a computer. The grid 16 can be rotatably mounted and rotated by a stepper motor by means of a lever. In this way, even the smallest angle changes can be set with sufficient accuracy. The parabolic mirror can be positioned in a similar manner.

Claims

claims

1. spectrometer (10) with a dispersing element (16) which is movable between at least two positions and in which in the first position the simply dispersed radiation (44) of a selected wavelength falls directly back into the incident beam path (42)

characterized in that

in the second position the dispersed radiation (32) of the selected one

Wavelength falls on a reflecting element (30) which is arranged in such a way that the radiation (34) can be deflected at least once more via the dispersing element (16) and then back into the incident steel path (38).

2. Spectrometer (10) according to claim 1, characterized by means for deflecting the beam from the dispersion plane such that the single-dispersed beam (44) runs in a different plane than the multi-dispersed beam (36).

3. Spectrometer (10) according to claim 2, characterized in that the means for deflecting the beam is a mirror (30) which is inclined about an axis (54) which is parallel to the dispersion plane and perpendicular to the incident beam (32) ,

4. Spectrometer (10) according to claim 2, characterized in that the means for deflecting the beam is a prism.

5. spectrometer (10) according to any one of the preceding claims, characterized in that the dispersing element is a grating (16).

6. Spectrometer (10) according to claim 5, characterized in that the grating (16) is an Echelle grating.

7. spectrometer (10) according to claim 6, characterized in that the blaze angle (Θ _{B) of} the Echelle grating (16) is at least 45 °.

8. spectrometer (10) according to any one of claims 5 to 7, characterized in that the positions of the grating (16) by the angle to the incident beam (38) are determined.

9. spectrometer (10) according to any one of the preceding claims, characterized in that the reflecting element is a mirror (30).

10. spectrometer (10) according to claim 9, insofar as it is related to claim 3, characterized in that the mirror (30) for reflecting the beam onto the dispersing element (16) also forms the mirror (30) for beam deflection from the dispersion plane ,

11. Spectrometer (10) according to claim 9 or 10, characterized in that the mirror (30) is a plane mirror.

12. Spectrometer (10) according to one of claims 1 to 8, characterized in that the reflecting element is a one-sided mirrored prism.

13. Spectrometer (10) according to one of the preceding claims, characterized in that the reflective element (30) is arranged such that the angle (β) that the perpendicular (22) to the dispersing element (16) with the dispersed beam (24) is smaller than the angle (α) with the incident

Beam (20).

14. Spectrometer (10) according to any one of the preceding claims, characterized in that the reflecting element (30) is arranged such that the angle (β) that the perpendicular (22) to the dispersing element (16) with the dispersed beam (22) is larger than the angle (α) with the incident

Beam (24).

15. Spectrometer (10) according to one of the preceding claims, characterized by means for controlling the angle at which the incident beam (20) falls on the dispersing element (16).

16. Spectrometer (10) according to claim 15, characterized in that are formed by a stepper motor with which a lever can be moved, with which a rotation of the dispersing element (16) can be effected.

17. Spectrometer (10) according to one of the preceding claims, characterized by means for controlling the angle (β) at which the dispersed beam (24) leaves the dispersing element (16).

18. Spectrometer (10) according to one of claims 15 to 17, characterized by

Means with which the positions of the components can be controlled from a computer.

19. Spectrometer according to one of the preceding claims, characterized in that an entry slit (12) is provided, and means with which the beam can be deflected such that the primary image (40) of this entry slit (12) at a selected wavelength in the dipersion plane is laterally offset next to the entry gap.

20. Spectrometer (10) according to any one of claims 1 to 19, characterized in that an entry slit (12) is provided, and means with which the beam can be deflected such that the primary image of this entry slit at a selected one Wavelength perpendicular to the dispersion plane is offset above or below the entrance slit.

21. Spectrometer (10) according to one of the preceding claims, characterized in that a re-magnification optics (68, 70) is provided for re-magnifying the primary image.

22. Spectrometer (10) according to claim 21, characterized in that the secondary magnification optics are formed by two cylindrical mirrors (68, 70).

23. Spectrometer (10) according to one of the preceding claims, characterized in that only reflective optical components (14, 30) are provided in the spectrometer (10) in addition to the dispersing element (16).

24. A method for measuring the radiation from a spectral range with different spectral resolution in which the beam is guided from an imaging optics (14) to a dispersing element (16) and from the dispersing element (16) back to the imaging optics (14) characterized in that the beam is directed by a change in position of the dispersing element onto a reflecting element (30), from which it is directed back onto the dispersing element (16).

25. A method for measuring the radiation from a spectral range according to claim 24, characterized in that the position change of the dispersing

Elements (16) is made by a rotation.

26. A method for measuring the radiation from a spectral range according to claim 24 or 25, characterized by the steps:

(a) Positioning the dispersing element (16) such that the dispersed beam of a selected wavelength travels back into itself (b) measuring the signal on a detector,

(c) positioning the dispersing element (16) such that the dispersed beam of a selected wavelength falls on the reflecting element (30), and

(d) Measuring the signal at the detector.

27. A method for measuring radiation from a spectral range according to claim

26, characterized in that after the second positioning of the dispersing element (16), the light of a reference light source is coupled into the spectrometer (10).

28. Use of a spectrometer (10) according to one of claims 1 to 23 for

Determination of the spectral properties of an excimer laser for photolithography.

29. Use of a spectrometer (10) according to claim 28, characterized in that the spectral properties of the excimer laser for its control and

Serve regulation.